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Date: Fri, 24 Jun 2016 15:35:44 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC/PATCH] ftrace: Reduce size of function graph entries
Hi Steve,
On Thu, Jun 23, 2016 at 09:37:40AM -0400, Steven Rostedt wrote:
> On Mon, 23 May 2016 00:26:15 +0900
> Namhyung Kim <namhyung@...nel.org> wrote:
>
> > Currently ftrace_graph_ent{,_entry} and ftrace_graph_ret{,_entry} struct
> > can have padding bytes at the end due to alignment in 64-bit data type.
> > As these data are recorded so frequently, those paddings waste
> > non-negligible space. As some archs can have efficient unaligned
> > accesses, reducing the alignment can save ~10% of data size:
> >
> > ftrace_graph_ent_entry: 24 -> 20
> > ftrace_graph_ret_entry: 48 -> 44
> >
> > Also I moved the 'overrun' field in struct ftrace_graph_ret to minimize
> > the padding. Tested on x86_64 only.
>
> I'd like to see this tested on other archs too.
>
> [ Added linux-arch so maybe other arch maintainers may know about this ]
Thanks, it'd be great if anyone could try this.
I think it doesn't affect most of (64-bit) archs since only x86_64,
arm64 and powerpc define CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS (and
it turns off CONFIG_HAVE_64BIT_ALIGNED_ACCESS). So other archs still
have (same) 8-byte alignment requirement.
Do 32-bit archs really require 64-bit alignment for unsigned long
long? IOW is it an alignment violation putting it in 32-bit boundary?
>
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > include/linux/ftrace.h | 16 ++++++++++++----
> > kernel/trace/trace.h | 11 +++++++++++
> > kernel/trace/trace_entries.h | 4 ++--
> > 3 files changed, 25 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index dea12a6e413b..35c523ba5c59 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -751,25 +751,33 @@ extern void ftrace_init(void);
> > static inline void ftrace_init(void) { }
> > #endif
> >
> > +#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
> > +# define FTRACE_ALIGNMENT 4
> > +#else
> > +# define FTRACE_ALIGNMENT 8
> > +#endif
>
> Swap the above. Having the #ifndef is more confusing to understand than
> to have a #ifdef.
Will do.
>
> > +
> > +#define FTRACE_ALIGN_DATA __attribute__((packed, aligned(FTRACE_ALIGNMENT)))
>
> Do we really need to pack it? I mean, just get rid of the hole (like
> you did with the movement of the overrun) and shouldn't the array be
> aligned normally without holes, if the arch can support it? Doesn't gcc
> take care of that?
I'm not sure I understood you correctly. AFAIK the size of struct is
a multiple of alignment unit and gcc manual says the aligment
attribute only can be increased unless the 'packed' is used as well..
Thanks,
Namhyung
>
> -- Steve
>
> > +
> > /*
> > * Structure that defines an entry function trace.
> > */
> > struct ftrace_graph_ent {
> > unsigned long func; /* Current function */
> > int depth;
> > -};
> > +} FTRACE_ALIGN_DATA;
> >
> > /*
> > * Structure that defines a return function trace.
> > */
> > struct ftrace_graph_ret {
> > unsigned long func; /* Current function */
> > - unsigned long long calltime;
> > - unsigned long long rettime;
> > /* Number of functions that overran the depth limit for current task */
> > unsigned long overrun;
> > + unsigned long long calltime;
> > + unsigned long long rettime;
> > int depth;
> > -};
> > +} FTRACE_ALIGN_DATA;
> >
> > /* Type of the callback handlers for tracing function graph*/
> > typedef void (*trace_func_graph_ret_t)(struct ftrace_graph_ret *); /* return */
> > diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> > index 5167c366d6b7..d2dd49ca55ee 100644
> > --- a/kernel/trace/trace.h
> > +++ b/kernel/trace/trace.h
> > @@ -80,6 +80,12 @@ enum trace_type {
> > FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> > filter)
> >
> > +#undef FTRACE_ENTRY_PACKED
> > +#define FTRACE_ENTRY_PACKED(name, struct_name, id, tstruct, print, \
> > + filter) \
> > + FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> > + filter) FTRACE_ALIGN_DATA
> > +
> > #include "trace_entries.h"
> >
> > /*
> > @@ -1600,6 +1606,11 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled);
> > #define FTRACE_ENTRY_DUP(call, struct_name, id, tstruct, print, filter) \
> > FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> > filter)
> > +#undef FTRACE_ENTRY_PACKED
> > +#define FTRACE_ENTRY_PACKED(call, struct_name, id, tstruct, print, filter) \
> > + FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> > + filter)
> > +
> > #include "trace_entries.h"
> >
> > #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_FUNCTION_TRACER)
> > diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
> > index ee7b94a4810a..5c30efcda5e6 100644
> > --- a/kernel/trace/trace_entries.h
> > +++ b/kernel/trace/trace_entries.h
> > @@ -72,7 +72,7 @@ FTRACE_ENTRY_REG(function, ftrace_entry,
> > );
> >
> > /* Function call entry */
> > -FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
> > +FTRACE_ENTRY_PACKED(funcgraph_entry, ftrace_graph_ent_entry,
> >
> > TRACE_GRAPH_ENT,
> >
> > @@ -88,7 +88,7 @@ FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
> > );
> >
> > /* Function return entry */
> > -FTRACE_ENTRY(funcgraph_exit, ftrace_graph_ret_entry,
> > +FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,
> >
> > TRACE_GRAPH_RET,
> >
>
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